Hypothetical reference decoder (HRD) conformance is typically a normative part of video compression standards and recommendations. A hypothetical reference decoder generally presents a set of requirements on the bitstream. A hypothetical reference decoder verifier may include software and/or hardware and is typically used to verify conformance of a bitstream to the requirements by examining the bitstream, detecting whether any hypothetical reference decoder errors exist and, if so, reporting such errors.
In the context of video compression standards, such as the International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) Moving Picture Experts Group-1 (MPEG-1) Standard, the ISO/IEC MPEG-2 Standard, the ISO/IEC MPEG-4 Standard, the International Telecommunication Union, Telecommunication Sector (ITU-T) H.261 Recommendation, the ITU-T H.263 Recommendation, and the ISO/IEC MPEG-4 Part 10 Advanced Video Coding (AVC) Standard/ITU-T H.264 Recommendation (hereinafter the “MPEG-4 AVC Standard”), a bitstream is determined to be conformant if the bitstream adheres to the syntactical and semantic rules embodied in the standard or recommendation. One such set of rules takes the form of a successful flow of the bitstream through a mathematical or hypothetical model of the decoder, which is conceptually connected to the output of an encoder and receives the bitstream from the encoder. Such a model decoder is referred to a hypothetical reference decoder in some standards or recommendations or the video buffer verifier (VBV) in other standards or recommendations. In other words, the hypothetical reference decoder specifies rules that bitstreams generated by a video encoder adhere to for such an encoder to be considered conformant under a given standard or recommendation. HRD is typically a normative part of video coding standards and, hence, any bitstream under a given standard has to adhere to the hypothetical reference decoder rules and constraints, and a real decoder can assume that such rules have been conformed with and such constraints have been met.
In the MPEG-4 AVC Standard, a hypothetical reference decoder and its operations are defined. The encoder is required to control the bit rate fluctuations so that the hypothetical reference decoder of a given buffer size would decode the video bit stream without suffering from buffer overflow or underflow.
Rate control algorithms with consideration given to hypothetical reference decoder compliance have been proposed previously. In these proposals, an upper bound of the bits for current picture is derived based on the current coded picture buffer (CPB) buffer status. While the cause of coded picture buffer underflow is the bits accumulated over multiple pictures prior to the underflow, restricting the bits for a single picture is inappropriate.
Digital video compression has been widely applied to TV broadcast, home entertainment, online streaming, and so forth. Depending on the application, a video bitstream can be coded using a constant bit rate (CBR) or a variable bit rate (VBR). With CBR, the bit rate of the bitstream is kept constant over a buffer period. The bits would be uniformly distributed to each picture regardless of the complexity of each picture. With VBR, the bit rate of the bitstream is not required to be constant. Bits can be allocated based on the complexity of the each picture, in order to stabilize picture quality. Both a constant bit rate and a variable bit rate require the coded picture buffer not to underflow or overflow. While the solution for a constant bit rate is straightforward, the solution for a variable bit rate can be complex since the bit allocation for a variable bit rate needs to consider both the picture complexity and the hypothetical reference decoder buffer constraints.